Abstract
Oncorhynchus mykiss is among the most extensively farmed fish species globally and simultaneously ranks as one of the most invasive aquatic taxa. Despite its non-native status in Europe, it remains a valued species in recreational fisheries and is frequently used to supplement streams with declining native Salmo trutta populations. In Slovakia, the presence of O. mykiss dates back to the late nineteenth century, with historical introductions into the Okna River basin and Lake Vihorlat occurring in the early twentieth century. Here, we present fish stock data and report the first documented occurrence of natural spawning redds of O. mykiss in the Okna River recorded in April 2025. This evidence confirms the existence of a self-reproducing, spring-spawning population, possibly descended from the original introductions. The spawning redds were located in gravel-sand substrates in shallow, low-flow areas after the period of rising water temperature and during stable discharge. Stock-assessment data indicate that O. mykiss is currently dominant over S. trutta , and comparisons with historical records suggest altered relative abundances of B. barbatula and Phoxinus sp. populations. These findings highlight, ongoing ecological interactions between non-native O. mykiss and native fish fauna in the Okna River.